Dear Joana,
Blood pressure is actually two readings. You will often here a reading such as 120/80. The first is the systolic (happens when blood is forcibly pumped out of the heart creating a pressure wave in the body arterial system.
The second is the diastolic blood pressure which happens as the heart relaxes. If you are referring to systolic blood pressure then the lowest 'normal' is usually 90. For diastolic blood pressure the lowest acceptable reading is 60. However, variability does exist and to answer more properly yes it is possible to attain lower blood pressures than this without serious decompensation.